This page is intended for technical skilled people. You need a PC being able to run DOS.
It is how to use the SOFIA Curve tracing Software (free download) yourself. Transconductance is not so much used for circuit design, but you do need to know gain and Rp. The SOFIA software is the best of it's kind, and it allows you to read the tube parameters from any point of the tube curves, even AFTER the curves are plotted. For this, you can just point the mouse to any point of the curves.
Most specially this will give you awareness, of how the dynamic parameters Gm, gain and Rp are changing with the operating point you choose. Given, that people have (unnecessary!) concerns about using a 3300 Ohms or 3500Ohms Ra transformer for the same tube, it is more important to know the true Rp this tube has in the operating point you choose. Reason is that you need good matching of Rp to Ra. However a tube with an Rp of 650 Ohms at one point, can have 900 Ohms at another point. So instead of worrying about the transformer Ra is 3300 Ohms or 3500 Ohms, which difference is only 6%, you should better know the Rp exactly in the individual operating point you choose. Differences can be 200% !
What follows here is a guide to install and how to use the SOFIA curve tracing software. However this is only when you know how to run DOS programs on your PC. For XP or lower, the PC can do it. For W7 or higher you need to install a DOS simulator program (freeware). You need no knowledge of DOS, since the program works the same as Windows. This software once belonged to the very exclusive Sofia tube tester, but it can be used also without the tester, when you have the tube curves filed at your PC disc. Then you can still do following things:
- Analyze a tube curve that was saved as a data file
- Zoom the tube curves as you need
The tube curves data file are in .PTE format, it is proprietary for the SOFIA tube tester. We supply you these files as a download. So in order to use the PTE files, you need to install the SOFIA software. The challenge is, this is a DOS program. You can install and use it normally on any PC with XP or lower. For this just open the open DOS window, and go ahead. You can install it, and use it under DOS, from the DOS window. Once it starts, it opens in full screen mode, and resolution and functionality is perfect.
How to work with DOS
WIndows7 was the first OS which could not run DOS programs. So if you have W7 or higher, or no Microsoft operating system, you need to install a DOS emulator, and once ot works, install the Sofia software under DOS. A nice Emulator one that works quick and easy is the 'DOSBOX'. If you want another, just check the internet for 'DOS Emulator freeware' and you'll find some more. When you get the black DOS box, it works, and via this you can install and install the SOFIA DOS Software. Once it is installed, it is enough to click on the sofia.exe file, or make a link to it that you can put on your PC desktop.
BELOW HERE are external links that we think are safe. However, we do not take responsibility for external links.
SOFIA SOFTWARE. Note: The Audiomatica company removed the free software download from their website, and then later put it back in. So that may repeat. Right now, you find it here: MANUAL - DESCRIPTION - SOFTWARE. If they renmove it againm you can still download it from the wayback engine. For this just select a recent site version like from 2011. You find SOFIA Software by clicking 'Techsupport', and then click 'discontinued products' If there are any problems, try another monts or year, but the most recents site copies are better. Link to Wayback Engine.
DOSBOX . Download here
DOSBOX is not needed for XP or lower. An XP System can run DOS by itself. For that click the 'START' box, and enter in the text box: cmd.exe.
Here is a screen shot of the SOFIA window, for the 300B-XLS tube. This tube is very rugged, and we let the tester go on, until 250mA and 600V. While testing, the data gets stored in the PTE file, and when finished the SOFIA SOFTWARE loads it into this window.
It looks to me, the Sofia software was inspired on what is probably the best analog curve tracer ever made, the Tektronix 576. Here testing a BC588B transistor.
When you look carefully at the above Sofia picture, you see there is a marker (cross hair) in the curves at 420V 75mA. You can just move the marker to where you want it. In the right part is displayed the exact position of the marker. (Below where it says MARKER). Then you also see: Gm = 6,29 Gain (u) is 5.02 and Rp=800 Ohms.
The effect comes now when you start to move the marker. In the next screen shot, you see the marker is moved to: 300V, 60mA. This is used by many as the 'standard' 300B operating point. Though you will probably never use this low point at only 18 Watt. Still we read from it, a bit more different 300B data: Gm=6,01 Gain(u) is 5,09 and Rp=850 Ohms
In the following picture, the cross hair was moved outside the chart, and this will give invalid results of the dynamic parameters. However this always represents a non-realistic working point anyway.
Here is how to use the software:
1) Under normal windows, place the PTE files, where the Sofia software expects it. This is difficult and individual, because you can create your own directories with the Sofia software. For the moment, go to the next step, but latest when need to select the PTE files, they should be saved on disc
2) Start the Sofia software. It looks like this:
Click on the 'ok' and it will tell you the hardware is not present.
Click 'ok' and you have a start window, which looks like this:
Click 'Disk'. Then 'CHDir' . Then you can choose the directory. It is HERE where your PTE files must be stored, so SOFIA can find them.
After you set have selected the directory, the start window comes again, and you choose from this window 'Measure' and then 'Anode'
Now comes this window, it is the function what we wanted, but there is not tube loaded yet. Try to move the marker with the mouse, and observe how the windows respond to it.
So the green screen, here above, you can always get, even without getting the data directory right in DOS. Only, to get any further from here, you need to the PTE files of the tubes that you are going to analyze, and put those in the right directory on your hard disk. (Simply by file manager, with Windows).
So the following part is done 'life' by using the PTE file of the 300B-XLS ==> Now click on the little 'folder' icon. It is in the top left. Choose the 300BXLS.PTE file. You have to download this from the EML website, and place it on your hard disk, at the position where you are going to select it with the SOFIA software.
Now come the curves if the tube we choose - Done!
Move the marker to where you want it. You can also zoom the curves with the arrow buttons.
QUALITY CONTROL WITH THE SOFIA SOFTWARE
Above here is a a 300B chart by Western Electric, which can be used for quality control. This chart is good, but using the SOFIA software is even better. At this chart you see transconductance as a function of the Control Grid Voltage. For come this may come as a surprise to see transconductance change so much. A new tube may have a transconductance of 1700 or 7200 and both is good, provided you use the right Control Grid Voltage and the right Anode voltage, same as in the chart.
Since transconductance loss is a very good way to judge remaining tube life, you can add the values in the chart. Here it was done for a Anode voltage of 350 Volt and a Control Grid Voltage of -77 Volt. (See note 1). After entering the tubes in this chart, you see they perform above nice. This chart type however ignores the Anode current. Also involve the Anode current would mean a three axis chart - which is possible, but so hard to display. However, the result by looking at Anode current, Anode voltage, and Control Grid Voltage (three parameters) at together, you can do with the SOFIA software. The result you can just read from the numeric windows at the right.
Here you see the marker set at Anode=460V and at a Control Grid Voltage estimated for the -115V curve. (see Note 1).
Now look in the top right window and read from there: 1,63mA/V (which means Gm=1630) . A perfect result, conforming to the WE data sheet, the blue dot at the left.
Here you see the marker set at Anode=150V and at a Control Grid Voltage estimated for the -18V curve (see Note 1).
Now look in the top right window and read from there: 7,23mA/V (which means Gm=7230) . A perfect result, conforming to the WE data sheet, the blue dot at the right.
|Note 1||This WE chart is for AC heating, so the Control Grid Voltage is referring to the AC method - so we need to take the wanted Control Grid Voltage, and reduce it by half the DC heater voltage.|
|Note 2||Test method of EML tubes|